Avalanche Photodiodes (APDs) are photodetectors that use avalanche multiplication to achieve internal gain. Single photon avalanche photodiodes (SPADs) are a specific class of avalanche photodiodes that are capable of detecting single photons.
Prior art APD arrays have also used various techniques for isolating adjacent APD elements. For example, PN junction isolation and mesa isolation are known in the prior art. PN junction isolation is generally achieved by confining the lateral extent of doping to separate p-type regions (on an n-type substrate) or n-type regions (on a p-type substrate) or both. Edge effects in isolated devices with positive bevel angles often results in electrical field crowding along the perimeter of the APD device, as illustrated in FIG. 43, which would normally cause a non-uniform avalanche gain profile. Edge effects in isolated devices are mitigated through the use of double-diffused structures, guard ring structures, or other approaches well known in the state of the art. See, for example, Y. Liu, S. R. Forrest, J. Hladky, M. J. Lange, G. H. Olsen, and D. E. Ackley, “A Planar InP/InGaAs Avalanche Photodiode with Floating Guard Ring and Double Diffused Junction,” J. Lightwave Technology, v. 10(2) February 2991, and Chapter 3: Breakdown Voltage in Power Semiconductor Devices, Pp. 67-127 by B. J. Baliga, PWS Publishing Company, Boston, Mass. 1996, which are hereby incorporated by reference.
Mesa isolation uses etching to remove semiconductor material from either the p-type region, the n-type region, or both regions of the device. Etching can consist of wet chemical etching using acidic or basic solutions, reactive ion etching, polishing, or any other technique that removes a portion of the semiconductor material between devices.
Another approach to isolating adjacent APD elements uses implant isolation to achieve a virtual positive bevel as described in U.S. Pat. No. 9,076,707, which is hereby incorporated by reference. In this case, etching is not used, but rather implant isolation is used to convert the region in the exterior of the virtual mesa from a highly conductive state to a lower conductivity state, as disclosed in U.S. Pat. No. 9,076,707. Implant isolation can be achieved by implanting a compensating dopant (such as implanting n-type dopant ions into a p-type region or implanting p-type dopant ions into a n-type region) or through the introduction of deep levels states. Deep level states can be achieved by the implantation of ions known to form deep level defects (e.g. oxygen into silicon), or by the implantation of neutral atoms (such as H+ or He+), where the implantation process causes crystalline damage to the semiconductor, introducing deep level states. Herein we will use the term implant isolation unless the application requires a specific type of implant isolation (compensating implant vs deep level implant).
For the specific case where the isolated devices operate near the condition of avalanche breakdown, one of the known techniques for isolating adjacent APD elements is to use a positive bevel mesa isolation, where the upper semiconductor layer is lightly doped (e.g., N−) and the lower semiconductor layer is highly doped (e.g., P++). There is a need for techniques to isolate adjacent devices operated near the condition of avalanche breakdown using a negative bevel, in which the upper semiconductor layer is highly doped (e.g., P++) and the lower semiconductor layer is lightly doped (e.g., N−).